1. Field of the Invention
The present invention relates to a vinyl polymer and to a method of manufacturing the same. More particularly, the present invention relates to a vinyl polymer that is suitable as a binder in a particulate magnetic recording medium, and to a method of manufacturing the same.
Still further, the present invention relates to a binder composition for a magnetic recording medium containing the above vinyl polymer, and to a magnetic recording medium and method of manufacturing the same.
2. Discussion of the Background
In recent years, means of transmitting information at high speed have become highly developed, permitting the transmission of data and images consisting of huge amounts of information. With advances in data transmission techniques, higher density recording capability has been demanded in recording media and recording and reproduction devices for recording, reproducing, and storing information.
Employing microparticulate magnetic powder, dispersing the microparticulate powder to a high degree, and increasing the smoothness of the surface of the magnetic layer are effective ways of achieving good electromagnetic characteristics in the high-density recording region. The method of incorporating a sulfonic acid (salt) group such as a SO3Na group into the binder is a known means of increasing the dispersibility of a microparticulate magnetic powder. In the present specification and the present invention, the term “sulfonic acid (salt) group” is used to include sulfonic acid groups and salts of sulfonic acid groups.
As methods of incorporating a sulfonic acid (salt) group into a vinyl polymer that is widely employed as a binder in magnetic recording media, Japanese Examined Patent Publication (KOKOKU) Heisei No. 1-26627 (Reference 1) proposes the combined use of starting material monomers in the form of a monomer containing a sulfonic acid (salt) group and a monomer containing an epoxy group, and using an addition reaction to incorporate the monomer containing the sulfonic acid (salt) group onto the monomer containing the epoxy group. Japanese Unexamined Patent Publication (KOKAI) Showa No. 60-238306 (Reference 2) proposes the method of introducing a sulfonic acid (salt) group by conducting an addition reaction with an epoxy group-containing vinyl chloride polymer in a water-based solvent. The contents of References 1 and 2 as well as their English language family member U.S. Pat. No. 4,707,411 are expressly incorporated herein by reference in their entirety.
The vinyl polymers obtained by the methods described in References 1 and 2 have sulfonic acid (salt) groups in the form of adsorbed functional groups as side chains. When these adsorb to the surface of the magnetic powder, it becomes possible to prevent aggregation of the magnetic powder and decreased dispersibility. However, the methods described in References 1 and 2 present the following problems in terms of synthesis.
As is described in Reference 1, when an addition reaction is conducted on a monomer containing an epoxy group, a bifunctional monomer is produced. When it is used in a polymerization reaction, a vinyl polymer with a multidimensional crosslinked structure is obtained that in turn causes gelling during polymerization and the like, making it difficult to control the reaction.
Reference 2 describes conducting a sulfonic acid (salt) group addition reaction in an aqueous system. However, in cases where crosslinking using an isocyanate curing agent is employed in the magnetic recording medium to increase the coating strength and the like, and a large amount of water is present, the isocyanate curing agent crosslinks with the water, compromising reactivity with the binder. Thus, it becomes difficult to obtain a strong coating despite using an isocyanate curing agent. Accordingly, binder resins that have been synthesized in an aqueous system such as that described in Reference 2 are not employed to prepare coating liquids for forming magnetic recording media as is, but require a drying step or the like, such as is done in Examples in Reference 2, to remove the water.
As set forth above, the methods of manufacturing vinyl polymers containing sulfonic acid (salt) groups that have been previously proposed present problems in terms of synthesis.